This invention relates to motor vehicle wheel end components and particularly to a wheel hub assembly having a disc brake rotor or drum incorporating novel means for attaching components of the assembly, and for machining braking surfaces of the rotor or drum.
Most motor vehicles today include disc brake systems for the front axle and many further include disc brakes at the rear axle positions. The disc brake rotor is a circular metal disc having opposed braking surfaces that are clamped by brake pads carried by a brake caliper to exert a braking effect. The wheel hub incorporates an anti-friction wheel bearing assembly in which one race of the bearing is coupled to the vehicle suspension and the other rotationally mounts the brake rotor and wheel. Ordinarily the rotating components of the rotor and hub assembly are manufactured separately and assembled together. This enables the brake rotor to be serviced and replaced if necessary during use. Moreover, the desired material characteristics for a brake rotor and hub components are different. Although efforts to integrate these components have been proposed, such an approach has not found wide-spread acceptance.
In order to enhance performance of the braking system it is desired to carefully and accurately control the dimensional characteristics of the rotor braking surfaces as the rotor rotates. The thickness variation of the disc and the lateral run-out or lateral deflection of the surfaces as they rotate needs to be held to minimum tolerances. Similarly, the radial run-out of the outer edges of the braking surfaces need to be controlled to ensure that the brake pads engage as much of the available rotor braking surface as possible without overlapping the edges of the rotor which gives rise to brake noise problems. The desire to control lateral and radial run-out of braking surfaces of a disc rotor are well known. However, manufacturers have faced difficulties in achieving enhanced control over these tolerances due to the influence of several factors.
Presently available manufacturing methods and designs of wheel end assemblies limit the accuracy to which radial and lateral run-out of braking surfaces can be controlled. One approach presently used is to separately finish machine the rotor and wheel hub, and later mate the two. The "stack up" of tolerance variations related to such an approach is significant. This is true not only because the machining operations of the parts are done separately, but in view of the fact that tight control over the relative positioning of the hub and rotor components is not provided. Brake rotors typically have a central hole which mounts over a protruding shoulder or post of the wheel hub. A number of wheel bolts projecting from the hub pass through clearance holes in the rotor. Due to the need to provide clearance between the bolts and associated bolt holes in the rotor, the rotor is not accurately positioned with respect to the hub surface. Slight angular variations in the relative position of these two parts can negatively impact run-out characteristics. Such variations occur in at least two ways. First, the center-to-center alignment of the rotor and hub is not well controlled. Second, the relative angular position or "clocking" of the components is also variable. In addition, the forces acting on the components as they are mounted to the vehicle cannot be readily duplicated during a machining operation. Thus, differences in deflections of the components between that occurring during machining operations and during use negatively contribute to run-out accuracies.
One approach to improving upon currently available brake rotor run-out accuracies is to assemble a rotor and hub and finish machine the rotor braking surfaces. This is done by employing accurately ground or machined surfaces of the hub as a datum for finishing the rotor braking surfaces. Although an improvement over prior processing, this approach is still limited by the lack of accuracy of control and fixing of the relative angular position of the hub and rotor. In this process, sheet metal nuts, such as so called "tinnerman" nuts are often used on the wheel mounting bolts to loosely connect the hub and rotor in an assembled condition. Such fasteners are used since they are thin and do not therefore interfere with the vehicle wheel mounted against the outboard rotor face. This approach however, suffers from some of the same shortcoming of the previously described method in that the relative angular position of the components cannot be assured and clamping loads acting on the parts during use cannot be readily duplicated.
In accordance with this invention, a novel wheel end assembly and method are provided which include special fastening means for connecting the two principle components together for machining operations. The fastening approach accurately controls the relative angular position and center-to-center alignment of the two principle components and simulates wheel mounting clamping forces. This fastening approach is preferably implemented in a manufacturing process in which these components are fastened together and thereafter the brake rotor braking surfaces are machined concurrently (or consecutively) with datum surfaces on the hub. A preferred locating surface is the cylindrical outer barrel of the hub which the wheel bearing assembly later mounts to. These retention fasteners are left in position to maintain the relative positions of the components as the vehicle is driven. Through this method and component design, extreme accuracy over lateral and radial run-out of the braking surfaces is provided.
In this invention, fasteners in the form of retention nuts are threaded onto the wheel mounting bolts and engage the rotor. The rotor and retention nuts define an interface which establishes the relative angular and center-to-center alignment of the hub and rotor. In a preferred embodiment, the nuts feature tapered outer surfaces which engage generally conical counter-bored surfaces of the rotor wheel bolt clearance holes. These fasteners provide a clamping force as well as accurately positioning the relative angular position of the hub and rotor. By maintaining the components in an assembled condition after machining the relative position of the parts is maintained and thus assembly, precise control over dimensional run-outs is provided.
The features of the present invention may also be implemented in connection with a drum brake unit incorporating a brake drum and hub assembly. Implementation of the invention is carried out in the same manner as in connection with the disc brake except that the brake surface constitutes inside cylindrical surface as opposed to the planer surface of a disc brake rotor. The advantages of controlling the dimensional accuracy of the braking surfaces are however, provided and are also desirable in drum brake applications.
Further objects, features and advantages of the invention will become apparent from a consideration of the following description and the appended claims when taken in connection with the accompanying drawings.